This invention relates to sprayable, rapid setting cementitious materials and to methods of making and applying such materials.
Cementitious materials are used extensively in the construction industry as pavements and/or surface coatings for infrastructures such as roadways, swimming pools, bridges, walls, tunnels and other structures which are exposed to the elements and/or to repeated heavy use. Many applications require rapid curing of the cementitious materials.
In a typical cement making and placing process, dry materials are added to a mixing chamber through an open hopper, to which additional dry material is fed as needed to maintain a continuous process. A pressurized gas stream, such as compressed air, is introduced into the mixing chamber to aerate, tumble and convey the dry materials through the mixing chamber. The gas stream is also used to blend a liquid component, also introduced under pressure, with the dry materials to form a slurry. The gas stream is typically introduced and maintained at a relatively low pressure (i.e., up to about 15 psi) through the use of various constricting devices, such as static mixers, venturi eductors and transvectors.
One technical problem that must be overcome is to avoid constriction of the mixing device. If constriction is excessive, the dry materials can be blown back into the material hopper, causing hazardous dusting conditions. Further, inadequate gas stream pressure can cause the slurry to clog the mixing equipment and impede movement of the slurry through the mixing chamber. Attempts to force the material through the dispensing equipment by increasing the gas stream pressure can fail from an inability to maintain sufficient pressure in the mixing chamber when it is open to ambient conditions.
U.S. Pat. No. 4,355,060 to Cornwell discloses a high temperature cementitious material comprising a mixture of magnesium oxide, fly ash, finely divided mineral aggregates and aqueous ammonium phosphate solutions. Cementitious material compositions which include ammonium phosphates, such as the composition disclosed in Cornwell '060, are characterized by an offensive odor that is produced during cure of the material and the resulting release of ammonia.
U.S. Pat. No. 4,390,371 to Cornwell discloses a method for mixing, spraying and placing such a cementitious material. The method uses static in-line mixers and an open hopper to hold blended dry material. The material is then transported with compressed air to the static mixing system, for example, by using a transvector or a venturi eductor.
Specialized equipment is necessary to make and place rapid setting, sprayable cementitious materials. In particular, the use of static in-line mixers for preparing and placing such materials can be problematic. Rapid setting compositions require faster mixing and faster transport through mixing and spraying equipment in order to avoid curing and hardening of the material while it is still in the equipment. Thus, these compositions must be mixed and moved through the equipment at relatively greater speeds and under relatively greater fluid pressures. Conventional cement mixing equipment is not designed to accommodate the increased fluid pressures required to mix and place these rapid setting compositions. For example, the use of air pressures greater than about 15 pounds per square inch (psi) in conventional processing equipment can cause problems such as leakage and mechanical seals, uncontrolled dusting of the dry ingredients, tunneling of compressed air through slurries and dry ingredients, and clogging of hoses with slurry or dry ingredients that are not adequately mixed or transported in the fluid stream. Further, the addition of fibrous reinforcing material, such as fiberglass, as a dry ingredient results in flocking or fluffing of the fiberglass within the static mixer and other interstices of the equipment. The result is incomplete mixing of the material and, consequently, nonuniformity and reduced strength of the final product.
The use of conventional air pressures to mix and place rapid setting cementitious compositions does not sufficiently propel either the dry material through the mixing equipment or the slurried material through a constricting device for satisfactory placement of the material. Further, only objects which are located immediately adjacent to the constricting device can be spray-coated using conventional air pressures.
It is therefore an object of the invention to provide an improved system for preparing and dispensing rapid setting cementitious materials. It is a further object of the invention to provide an improved rapid setting cementitious composition. Another object of the invention is to provide an apparatus for making and placing rapid setting cementitious materials. These and other objects will be apparent from the description which follows.